nofars.net

By Ross Goodall, WD4NJV

Most of our VHF radios have the capability to receive the VHF aircraft band which runs between 108.00 and 137.00 MHz.   AM is used worldwide on VHF and UHF.  Upper sideband is used for all HF communications.

Different VHF band sections are used for navigational aids and traffic control.   108 to 117.95 MHz and 118 to 136.975 MHz use 760 channels spaced 25 KHz apart. In some European countries this spectrum is divided into 8.33 kHz segments allowing 2,800 channels.   

AM is used because it allows stronger stations to override weaker or interfering stations. If FM were used, the capture effect would cause only the stronger station to be heard and the weaker station with urgent information not to be heard.  There would be an enormous cost of retrofitting all legacy VHF radios to convert to FM or digital. The typical VHF transmission range at 35,000 feet altitude is 200 miles.

Audio quality for AM is limited to bandwidth.  The largest bandwidth on VHF is limited to 8.333 KHz so the highest possible audio frequency is 4.166 kHz which is sufficient for voice.  Although the audio frequency range is low as compared to the human hearing range, it is sufficient for speech.

Most private pilots who fly regularly and commercial pilots prefer to provide their own headset.

Digital radio would increase the capacity of channels and reduce possible interference and jamming.   International cooperation and change out costs for existing equipment would have to be considered in the changeover.  With digital it would be impossible for one station to cut into an emergency transmission.

It is illegal to transmit on Airband frequencies in most countries, without a suitable license, although an individual operator license may not be required.  FM broadcasters must make certain that there are no spurious signals transmitted in the Aircraft Band. In some foreign countries it is illegal to listen to Airband transmissions.  1

Icom and Yaesu also make handheld aircraft specific radios.

There are numerous You Tube videos covering air traffic control, aircraft radio basics. You can listen on the internet to scanner feeds of air traffic control (ATC) from stations all over the world.

Aircraft band radio calls follow the format: 

1. Who are you calling (clearance, ground, tower, approach/departure),

2. Who are you? (type of aircraft and tail number,

3. Where are you (position in air or airport). 

4. What do you want? (IFR clearance, taxi instructions, takeoff clearance, landing clearance.

5. Any other important information.  2

The emergency channel is 121.5 MHz AM

References:
1.    Wikipedia: Airband

2     YouTube: Aircraft Communications and Radio Procedures

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